1. Field of the Invention
The present invention relates to a switching mode power supply (SMPS), and a method for outputting voltage therefrom. More particularly, the present invention relates to a switching mode power supply that can be miniaturized, and effective attenuation of electromagnetic wave noise generated from a switching transistor.
2. Description of the Related Art
Generally, stable output voltage can be obtained from a switching mode power supply (SMPS) by adjusting operation time of a switching transistor connected to a first side coil of a transformer with a pulse width modulation Integrated Circuit (IC) even though the output voltage fluctuates.
FIG. 1 is a circuit diagram showing a conventional switching mode power supply. As shown in the drawing, the conventional switching mode power supply includes a power source 10, a filter 20, a first side rectifier 30, a capacitor 35, a transformer 40, a second side rectifier 50, a switching transistor 70, a heat sink 80, and a controller 60.
When an alternating current (AC) power is inputted from the power source 10 into the switching mode power supply and the switching transistor 70 is turned on upon receipt of an operation signal from the controller 60, the AC power is filtered in the filter 20 and then rectified and planarized into a predetermined level of power through the first side rectifier 30 and the capacitor 35. When the resultant current is provided to the first side coil of the transformer 40, voltage is generated and, accordingly, an induced current is generated in a second side coil of the transformer 40. After a predetermined time passes, the switching transistor 70 is turned off and the induced current generated in the second side coil of the transformer 40 is rectified in the second side rectifier 50 to thereby output a direct current (DC) voltage.
Since the switching transistor 70 of the switching mode power supply performs the switching operation at a very high speed due to high frequency pulse signals, strong electromagnetic wave noise is generated and radiated out into the adjacent area of the switching transistor 70 during the operation of the switching transistor 70. In order to remove the noise of the switching transistor 70, the heat sink 80 is set up in the adjacent area of the switching transistor 70. Although the heat sink 80 is electrically insulated from the switching transistor 70, it is physically connected to it. Thus, it transforms the electromagnetic waves from the switching transistor 70 into heat and radiates the heat.
The metallic heat sink 80, however, tends to amplify the electromagnetic waves from the switching transistor 70. To improve on this disadvantageous property, the electromagnetic waves are reduced by grounding the heat sink 80. In this case, however, a voltage difference is caused between the heat sink 80 and other elements in the first side including the switching transistor 70. Thus, when the heat sink 80 is set up adjacently to the elements in the first side, a short may occur due to contact. Therefore, the heat sink 80 should be set up at a predetermined distance from the elements in the first side. Since this increases the size of required space for setting up elements, it is hard to miniaturize the power supply.
An alternative method is known where the heat sink 80 is made to float to miniaturize the power supply, the floating heat sink 80 has a shortcoming that it cannot decrease the electromagnetic wave noise.
Therefore, a method and configuration that can not only miniature the power supply but also attenuate the electromagnetic wave noise of the switching transistor 70 are desired.